Electromagnetic interference suppression circuits are known and are commonly used for suppressing conducted electromagnetic interference signals present on a signal or power line. These conducted electromagnetic interference signals can be classified into differential mode electromagnetic interference signals, which are conducted on the power supply line and the power return line in opposite directions, and common mode electromagnetic interference signals, which are conducted on the power supply line and the power return line in the same direction.
In theory, an electromagnetic interference suppression circuit is able to generate a suppression signal that can reduce or even eliminate the unwanted electromagnetic interference signal. For the best suppression performance, the generated suppression signal should be a negative of the electromagnetic interference signal (e.g., equal in magnitude and anti-phase) so that the two always completely cancel each other.
In practice, however, a substantial or complete cancellation can be difficult to achieve. First, a gain factor of the circuit has to be precisely controlled to be equal to unity to match the amplitude of the suppression signal with the amplitude of the electromagnetic interference signal. But such gain factor is affected by various factors including the environment in which the circuit is arranged, intrinsic parameters of components of the circuit, the deterioration of circuit components over time, the operation condition of the circuit, etc. These factors cause undesirable performance variation during production as well as throughout the operating life of the circuit. Second, the suppression signal has to be precisely controlled to be about 180 degrees out of phase with the electromagnetic interference signal voltage (when the suppression signal is a voltage signal), or substantially in phase with the electromagnetic interference signal current (when the suppression signal is a current signal). Another difficulty with the implementation of electromagnetic interference suppression circuits relates to feedback control. In particular, the use of amplifiers or like circuit components with high gain and good phase characteristic would easily introduce undesirable side effects such as instability and oscillation to the circuit.